Here's the sign comemmorating the massive floods of January 1997. Check the following Youtube link to see the carnage from that week as viewed from NPS video cameras. http://www.youtube.com/watch?v=GttPGfZgY2w
Or follow this link to see Yosemite Association Photos of the flood:
http://www.yosemite.org/vryos/pages/views/floodindex.htm